You can't smoke on airplanes. For many reasons—both social and safety-related—that's a great idea, even if it does make some people a little cranky over long flights. So why the hell do they install ashtrays in the toilets?

You might think that they're a hangover, from more liberal days, on planes yet to be replaced—but you'd be wrong. Smoking has been entirely banned on planes for a long old time, so we'd have seen changes by now if that were the case. In fact, it turns out that ashtrays in airplane lavatories are mandatory. From the Code of Federal Regulations for airworthiness:

"Regardless of whether smoking is allowed in any other part of the airplane, lavatories must have self-contained, removable ashtrays located conspicuously on or near the entry side of each lavatory door, except that one ashtray may serve more than one lavatory door if the ashtray can be seen readily from the cabin side of each lavatory served."

What. The. Hell? If that sounds incredibly crazy to you, don't worry: there is an explanation for why your plane can't leave the terminal without an ashtray onboard. Matt Simmons has done some digging and, writing on his blog, he explains:

"Back in 1973, a flight crashed and killed 123 people, and the reason for the crash was attributed to a cigarette that was improperly disposed of.

"The FAA has decided that some people (despite the policies against smoking, the warning placards, the smoke detector, and the flight attendants) will smoke anyway, and when they do, there had better be a good place to put that cigarette butt."